Generally, a moving image is encoded when transmitted or stored to increase the transmission efficiency and the compression efficiency. Upon receipt of the encoded moving image, the receiving side decodes the image, and performs sharpening on the decoded image to reproduce it. If an encoded moving image is decoded and sharpened in this manner, the sharpening may increase the coding distortion and may result in image quality degradation.
For example, Japanese Patent Application Publication (KOKAI) No. H9-81104 discloses a conventional technology for preventing such image quality degradation. According to the conventional technology, a gain and a frequency for contour enhancement can be varied depending on the quantization level to prevent the image quality degradation.
In the conventional technology, only the gain and the frequency for contour enhancement are varied. This is not sufficient to reduce the sharpening effect, and cannot sufficiently reduce an increase in the coding distortion due to the sharpening.